This invention relates to a fluidized bed reactor, and, more particularly, to such a reactor in which the rate of air flow to the fluidized bed of the reactor is measured.
Fluidized bed reactors, including steam generators, combustors and gasifiers, are well known. In these arrangements, air is passed through a perforated plate, or the like, which supports a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. When the heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator, the fluidized bed system offers an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed combustion system is commonly referred to as a bubbling fluidized bed in which a bed of particulate material is supported by an air distribution plate. Combustion-supporting air is introduced to the bed of particulate material through a plurality of perforations in the plate, causing the material to expand and take on a suspended or fluidized state. In the event the reactor is in the form of a steam generator, the walls of the reactor are formed by a plurality of heat transfer tubes. The heat produced by combustion within the fluidized bed is transferred to a heat exchange medium, such as water, circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, for separating water from the steam thus formed which is routed to a turbine to generate electricity or to a steam user.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turn down afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a fast, or circulating, fluidized bed. According to this technique, fluidized bed densities between 5% and 20% volume of solids are attained which is well below the 30% volume of solids typical of the bubbling fluidized bed. The formation of the low density circulating fluidized bed is due to its small particle size and to a high solids throughput, which requires high solids recycle. The velocity range of a circulating fluidized bed is between the solids terminal, or free fall, velocity and a velocity which is a function of the throughput, beyond which the bed would be converted into a pneumatic transport line.
The high solids circulation required by the circulating fluidized bed makes it insensitive to fuel heat release patterns, thus minimizing the variation of the temperature within the combustor or gasifier, and therefore decreasing the nitrogen oxides formation. Also, the high solids loading improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle. The resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption. Furthermore, the circulating fluidized bed inherently has more turndown than the bubbling fluidized bed.
In both a bubbling fluidized bed reactor and a circulating fluidized bed reactor, the bed of particulate material is supported by an air distribution plate. An air plenum is located below the air distribution plate and the plenum receives air from a circulation device such as a fan. The combustion supporting air is passed from the air plenum through openings or perforations in the air distribution plate to fluidize the particulate material. is important that the air flow to the bed of particulate material be measured to ensure that the flow of air to the bed is sufficient to maintain the fluidized state of the bed and, in the case of a circulating fluidized bed, to ensure the bed is not converted to a pneumatic transport line. The conventional methods of measuring air flow to the bed of particulate material, however, present many disadvantages and drawbacks. For example, a first conventional method requires placing an orifice plate or venturi in a long section of ducting between the fan and the plenum, negating the possibility of a compact reactor design, while a second conventional method requires the utilization of a hot wire anemometer or a honeycomb device which greatly adds to the capital expense and operating costs of the reactor.